Electrical and structural characterization of AlGaN/GaN field-effect transistors with recessed gate

Performance of AlGaN/GaN heterostructure field-effect transistors (HFETs) with recessed gate was investigated and compared with non-recessed counterparts. Optimal dry etch conditions by plasma assisted Ar sputtering were found for ~6 nm gate recess of a 20 nm thick AlGaN barrier layer. A decrease of the residual strain after the gate recessing (from -0.9 GPa to -0.68 GPa) was evaluated from the photoluminescence measurement. The saturation drain current at the gate voltage VG = 1 V decreased from 1.05 A/mm to 0.85 A/mm after the recessing. The gate voltage for a maximal transconductance (240-250 mS/mm) has shifted from -3 V for non-recessed HFETs to -0.2 V for recessed counterparts. Similarly, the threshold voltage increased after the gate recessing. A decrease of the sheet charge density from 1 x 10 13 cm-2 to4 x 10 12 cm-2 at VG = 0 V has been evaluated from the capacitance measurements. The RF measurements yielded a slight increase of the cut-off frequencies after the gate recessing. All these indicate that the gate recessing is a useful tool to optimize the AlGaN/GaN HFET performance for high-frequency applications as well as for the preparation of normally-off devices.